Early life
Born at Newtown Park, near Dublin, he was the eldest son of John Hartpole Lecky, a landowner. He was educated at Kingstown, Armagh, at Cheltenham College, and at Trinity College, Dublin, where he graduated B.A. in 1859 and M.A. in 1863, and where, with a view to becoming a clergyman in the Irish Protestant Church, he studied divinity.

Career
In 1860 he published anonymously a small book entitled The Religious Tendencies of the Age, but on leaving college he abandoned his original intention and turned to historiography. In 1861 he published Leaders of Public Opinion in Ireland, a brief sketch of the lives and work of Jonathan Swift, Henry Flood, Henry Grattan and Daniel O'Connell, which showed great promise. This book, originally published anonymously, was republished in 1871; and the essay on Swift, rewritten and amplified, appeared again in 1897 as an introduction to a new edition of Swift's works.

Two learned surveys of certain aspects of history followed: A History of the Rise and Influence of Rationalism in Europe (2 vols., 1865), and A History of European Morals from Augustus to Charlemagne (2 vols., 1869). The latter aroused criticism, with its opening dissertation on "the natural history of morals," but both have been generally accepted as acute and suggestive commentaries upon a wide range of facts.

Lecky then devoted himself to the chief work of his life, A History of England during the Eighteenth Century, vols. i. and ii. of which appeared in 1878, and vols. vii. and viii. (completing the work) in 1890. His object was "to disengage from the great mass of facts those which relate to the permanent forces of the nation, or which indicate some of the more enduring features of national life". In carrying out this task, Lecky displays many of the qualities of a great historian.

The work is lucid in style, extensive in its use of source material, and, above all, by impartial throughout. These qualities are particularly valuable in the chapters dealing with the history of Ireland, and in the "cabinet" edition of 1892, in 12 volumes (frequently reprinted), this part of the work is separated from the rest, and occupies five volumes under the title of A History of Ireland in the Eighteenth Century.

A volume of Poems (1891) was less successful. In 1896 he published two volumes entitled Democracy and Liberty, in which he considered, with special reference to Britain, France and America, some of the tendencies of modern democracies. The pessimistic conclusions at which he arrived provoked criticism both in Britain and America, which was renewed when he published in a new edition (1899) an elaborate and very depreciatory estimate of Gladstone, then recently dead. This work, though essentially different from the author's purely historical writings, has many of their merits, though it was inevitable that other minds should take a different view of the evidence.

In The Map of Life (1900) he discussed in a popular style some of the ethical problems which arise in everyday life. In 1903 he published a revised and greatly enlarged edition of Leaders of Public Opinion in Ireland, in two volumes, from which the essay on Swift was omitted and that on O'Connell was expanded into a complete biography of the great advocate of repeal of the Union. Though always a keen sympathizer with the Irish people in their misfortunes and aspirations, and though he had criticized severely the methods by which the Act of Union was passed, Lecky, who grew up as a moderate Liberal, was from the first strenuously opposed to Gladstone's policy of Home Rule, and in 1895 he was returned to parliament as Unionist member for Dublin University. In 1897 he was made a privy councillor, and among the coronation honours in 1902 he was nominated an original member of the new Order of Merit.

Degrees
His university honours included the degree of LL.D. from Dublin, St Andrews and Glasgow, the degree of D.C.L. from Oxford and the degree of Litt.D. from Cambridge. In 1894 he was elected corresponding member of the Institute of France. He contributed occasionally to periodical literature, and two of his addresses, The Political Value of History (1892) and The Empire, its Value and its Growth (1893), were published.

Family
He married in 1871 Elizabeth, baroness de Dedem, daughter of baron de Dedem, a general in the Dutch service, but had no children. She contributed articles, chiefly on historical and political subjects, to various reviews. A volume of Lecky's Historical and Political Essays was published posthumously (London, 1908).